Phospholipids help transport fat between gut and liver in the form of micelles. When phospholipids are added to water, they self-assemble into sphere-like structures with the hydrophilic regions facing the outside, completely shielding the hydrophobic regions. Since the blood is about 90% water, fats have to be assembled into micelles before they can be transported in large quantities from the alimentary canal to the liver, via the hepatic portal vein. Lipids are transported to the liver so as to maintain body homeostasis, to ensure that the level of lipids in the blood at any point in time would not be too high or too low. Lipids are also transported to the liver to serve as raw materials for the synthesis of cholesterol and lipoproteins.
Triglycerides are large molecules comprising of 3 fatty acid molecules joined to a glycerol by an ester linkage and it serves an important function as a storage lipid. This is made possible as it is non-polar and hence insoluble in water. It is an excellent source and store of energy. Triglycerides store more than twice the amount of energy (38kJ/mol) as compared to carbohydrates and proteins (17kJ/mol). This is also because of the high calorific value of triglycerides, as a given mass of lipid yields more than twice the amount of energy on oxidation than an equal mass of carbohydrates. The reason for this is that lipids have a higher proportion of hydrogen and an almost insignificant proportion of oxygen as compared to carbohydrates. Fatty acid chains are broken down 2 carbons at one time and converted into glycerol, which is then converted into acetyl-coA and incorporated into the Krebs cycle of cellular respiration. Each molecule of glycerol forms as many ATPs as half a molecule of glucose.
Triglycerides are stored just beneath the skin in adipose tissues and also surrounding the organs. Subcutaneous fats serve as heat insulators for animals living in cold climates, as lipid are poor conductors of heat. This is especially important for animals living in the Artic climate like the polar bears. Fats in adipose tissues surrounding vital organs such as the heart, eyes, and kidneys, serve to protect the organs from damages. On oxidation of triglycerides, water is formed as a by-product. Desert animals such as the kangaroo rat stores fat as a source of water.
Cholesterol is an important class of lipids for maintaining membrane fluidity. It spans only one layer of the phospholipid bilayer and is absent from the inner membranes of the chloroplast and the mitochondria. A cholesterol molecule is hydrogen-bonded to a phospholipid molecule in the cell membrane. Cholesterol has the dual effect of increasing membrane fluidity at low temperature, and decreasing it at high temperatures. The intercalation of cholesterol molecules into the lipid monolayers results in reduced membrane fluidity at high temperatures as cholesterol prevents close-packing of the phospholipid molecules. At lower temperatures, cholesterol prevents the hydrocarbon chains of the phospholipids from aggregating as temperature is decreased, thereby reducing the tendency of membranes to freeze upon cooling. Apart from maintaining membrane fluidity, cholesterol decreases the permeability of a lipid bilayer to ions and small polar molecules by filling in the spaces between hydrocarbon chains of membrane phospholipids. By doing so, it plugs the small channels through which diffusion of ions and small polar molecules may occur. Cholesterol also prevents cell lysis to a certain extent as it enhances both the flexibility and mechanical stability of the phospholipid bilayer. It immobilizes the first few hydrocarbon groups of the phospholipid molecules, making the bilayer less deformable.
Cholesterol is also an important biological molecule as it is a precursor for the synthesis of the steroid hormones and bile acids. It is a key intermediate in the synthesis of sex hormones such as estrogen and testosterone. Both dietary cholesterol and that synthesized by the liver are carried in the blood in the form of lipoprotein particles. It can be in the form LDL (low density lipoprotein), which is derived from saturated fats and may be deposited in arteries, causing atherosclerosis, or in the form of HDL (high density lipoprotein), which is derived from unsaturated fats, and has the ability to pick up cholesterol that has been deposited in the arteries as plaque. HDL 'dissolves' the deposit and transports it to the liver where it is removed from the bloodstream. Increasing levels of HDL helps to lower the concentration of 'bad' LDL in the blood, thus reducing the chance of build-up and heart disease. Cholesterol can be excreted in the bile as free cholesterol or as bile salts following conversion to bile acids in the liver. Bile acids perform four physiologically significant functions:
- Their synthesis and subsequent excretion in the feces represent the only significant mechanism for the elimination of excess cholesterol.
- Bile acids and phospholipids solubilize cholesterol in the bile, thereby preventing the precipitation of cholesterol in the gallbladder.
- They facilitate the digestion of dietary triglycerides by acting as emulsifying agents that render fats accessible to pancreatic lipases.
- They facilitate the intestinal absorption of fat-soluble vitamins.
Phospholipids associate with hydrophilic oligosaccharides to form glycolipids, which plays an important role in cell recognition. Cell recognition is the cell’s ability to distinguish one type of cells from another. It is important in the sorting of cells into tissues and organs and it is the basis for the rejection of foreign cells by the immune system. Cell recognition ensures that the antibodies in out body will attach foreign bacteria and germs and not our own cells. It also prevents auto-immune disease where the immune system destroys our own cells. Glycolipids also contribute to membrane stability, because as they are highly hydrophilic, the sugars help to orientate the glycolipids in the membrane so that they are kept in contact with the external aqueous environment and are unlikely to rotate towards the interior or diffuse transversely.
The sphingolipids, like the phospholipids, are composed of a polar head group and two nonpolar tails. The core of sphingolipids is the long-chain amino alcohol, sphingosine. The sphingolipids include sphingomyelins and glycosphingolipids. Sphingomyelins are the only sphingolipid that are also phospholipids. Sphingolipids are a component of all membranes but are particularly abundant in the myelin sheath.
Waxes are similar to fats and oils but their long-chained fatty acids are linked to a long-chained alcohol. Hence, it is totally insoluble in water and is chemically inert. In plants, it forms a waxy cuticle that covers the epidermis of the leaves and stems to prevent excessive loss of water through evaporation and cuticular transpiration. It limits the diffusion of water into the plant that could affect the water potential of the leaf, and it protects the leaf from parasites to a certain extent. In insects, they also have a layer of waxy cuticle to help cut down on water loss. Bee wax is an abdominal secretion of bees and they use it to hive their cells.
